Bias sputtered NbN and superconducting nanowire devices
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Dane, Andrew E. et al. "Bias sputtered NbN and superconducting
nanowire devices." Applied Physics Letters 111, 12 (September
2017): 122601.
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http://dx.doi.org/10.1063/1.4990066
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AIP Publishing
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Final published version
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https://hdl.handle.net/1721.1/126227
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Appl. Phys. Lett. 111, 122601 (2017); https://doi.org/10.1063/1.4990066 111, 122601
© 2017 Author(s).
Bias sputtered NbN and superconducting
nanowire devices
Cite as: Appl. Phys. Lett. 111, 122601 (2017); https://doi.org/10.1063/1.4990066
Submitted: 13 June 2017 . Accepted: 06 September 2017 . Published Online: 18 September 2017
Andrew E. Dane, Adam N. McCaughan, Di Zhu , Qingyuan Zhao, Chung-Soo Kim, Niccolo Calandri , Akshay Agarwal, Francesco Bellei, and Karl K. Berggren
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Single-photon detectors combining high efficiency, high detection rates, and ultra-high timing resolution
Bias sputtered NbN and superconducting nanowire devices
Andrew E.Dane,1Adam N.McCaughan,1,2DiZhu,1QingyuanZhao,1Chung-SooKim,1
NiccoloCalandri,1,3AkshayAgarwal,1FrancescoBellei,1and Karl K.Berggren1
1
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
2
National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA 3
Dipartimento di Elettronica, Informazione e Bioingegneria, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
(Received 13 June 2017; accepted 6 September 2017; published online 18 September 2017) Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quan-tum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as supe-rior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speak-ing, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (Tc) values than typical polycrystalline materials. Here, we demon-strate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO2has a lowTcand high resistivity but is too rough for fabricating unconstricted nano-wires, andTcis too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, theTcof the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with aTcof 7.8 K and a resis-tivity of 253 lX cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to satu-rate at bias currents approaching the critical current, indicating that the device’s quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nano-cryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4990066]
In a variety of emerging quantum- and nano-devices, per-formance is limited by material synthesis and device fabrication. These limitations are especially evident in sensitive photon detectors1and quantum computing elements.2Superconducting nanowire single photon detectors (SNSPDs)3promise to com-bine near 100% detection efficiency4 with >100 megacounts per second, picosecond timing jitter,5 and sensitivity ranging from x-rays to mid-infrared wavelengths.6However, this prom-ise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The link between SNSPD performance and material properties is still being studied.7The addition of amorphous superconductors to the palette of demon-strated SNSPD materials has advanced the state of the art. The short time between the introduction of WSi8 and the leap in maximum demonstrated single-pixel system detection effi-ciency9indicates that substantial opportunity exists for improv-ing SNSPDs by engineerimprov-ing the materials used to make them.
The central problem that we hope to address through tai-loring superconducting niobium nitride (NbN) for SNSPDs is that the highest single-pixel detection efficiency and the deepest saturation have been achieved in SNSPDs based on
amorphous materials,4while the lowest timing jitter and high-est counting rates were demonstrated in devices made from polycrystalline materials.5Our goal was to combine the detec-tion efficiency performance of the amorphous detectors with the timing performance and operating temperature of the poly-crystalline ones. We employed an empirical approach for the fabrication a material suitable for this task because despite gains in our understanding about how SNSPDs work,10,11 a full description of the detector operation is still being devel-oped.12Whether it is possible to combine the best properties of these two material systems into one device or whether there exists an intrinsic trade-off between the detection and timing performance remains to be seen.
The superconducting and normal-state properties of NbN films are affected by the stochiometry,13 crystal phase,14impurity content,15disorder,16thickness,17and car-rier concentration,18 all of which can be influenced by the deposition method. Ultrathin NbN films are commonly pre-pared by reactive sputtering of a niobium target in a mixture of argon and nitrogen (N2) gases. Amorphous substrates have been used when depositing sputtered films; however,
0003-6951/2017/111(12)/122601/5/$30.00 111, 122601-1 Published by AIP Publishing.
films prepared on crystalline substrates suitable for epitaxy show higher superconducting critical temperature (Tc) and lower resistivity at the same thicknesses.19 Intentional heat-ing of the substrate to temperatures of a few-hundred degrees Celsius or more has been used to improve the crystalline properties of the deposited film, although ambient tempera-tures combined with lattice-matched substrates have demon-strated 5-nm-thick films, a thickness relevant for SNSPDs, with 12 KTc.
13
In this work, we show that the addition of ion bombard-ment during the sputter deposition of ultrathin NbN films onto unheated, thermally oxidized silicon can increase film Tcand reduce film resistivity and that the resulting material can be fabricated into SNSPDs with saturated internal effi-ciency and a jitter of 38 ps at a temperature of 2.5 K. Following the suggestion that a reduced superconducting energy gap may lead to improved detection efficiency,20we modified the typical sputter deposition method used to make ultrathin films of NbN to reduce the filmTc. By removing the usual substrate heating to800C, both the NbN filmTc and the maximum grain size (see supplementary material) were reduced. However, 5-nm-thick films deposited at ambi-ent temperature have critical temperatures that we believe are too small to yield devices with10 ps jitter while operat-ing at liquid helium temperatures, and thicker unheated NbN films have substantial surface roughness which would likely lead to constrictions.10 Both of these issues were addressed by the addition of ion bombardment due to substrate biasing during deposition. Using bias sputtered NbN films, we dem-onstrated SNSPDs and nanocryotrons (nTrons).21This mate-rial was also used to fabricate a large-area single photon imager, reported elsewhere.22
The experimental work of this letter can be divided into three parts: (1) the characterization of NbN sputter tion rates under different chamber conditions, (2) the deposi-tion and electrical characterizadeposi-tion of 5-nm-thick NbN films, and (3) fabrication and testing of SNSPDs and nTrons based on films prepared in two steps. To compare 5-nm-thick films deposited under different chamber conditions, changes to the film deposition rate had to be measured and then compen-sated for. Relatively thick films (20–30 nm) were depos-ited, and their thicknesses were measured using x-ray reflectivity (XRR) to determine the deposition rate for each of the conditions studied. At each chamber condition, the sputtering time was subsequently adjusted to yield 5-nm-thick films. Electrical characterization of the thin films included four-point probe measurements of filmTcand sheet resistance. SNSPDs and nTrons were fabricated by optical and electron beam lithography. SNSPDs were measured with a 1550 nm photon input at 2.5 K.
All films discussed in this work were deposited in a ded-icated AJA International Inc Orion series sputtering system by DC reactive magnetron sputtering using a sputter gun source, in a cryopumped chamber with a typical base pres-sure of 6.7 10 7Pa (5 10 9Torr). The DC current was set to 400 mA, and the sputtering pressure was 3.3 10 1Pa (2.5 10 3Torr) for all depositions. The flow rate of Ar was 26.5 cm3/min, while the N2 flow was varied as shown. An RF power of 14 W was applied to the 4-in. diameter sample holder from an external RF source and matching network in
order to sputter-clean samples prior to deposition. For bias sputtered samples, the RF power was increased to 50 W dur-ing the deposition step of the process which caused a DC voltage of approximately 280 V to develop in the sample holder. Otherwise, the sample bias was turned off during the sputtering step. No intentional heating, via heat lamps or heater elements, was added. Additional details are provided in thesupplementary material.
Altering the chamber conditions in order to tune theTc of reactively sputtered NbN has the unwanted effect of changing the film deposition rate23and makes it more diffi-cult to judge which films will make the best SNSPDs because the film Tc is a strong function of thickness in the few-nanometer regime, approaching the superconductor-insulator transition.17 Our initial attempts to deposit few-nanometer-thick NbN without heating resulted in films with Tcvalues of less than 5 K, less than half of what we observed while heating to 800C.24 The relative amount of nitrogen and niobium in NbN films is a primary influence on the film Tc and resistivity.
25,26
Therefore, to increase the Tc of the films deposited at ambient temperature, we varied the rela-tive ratio of nitrogen and niobium in the film by changing the nitrogen flow rate. Additionally, we found that by adding a 50 W RF bias to the substrate holder, we could almost dou-ble the observedTc. Initially, it was unclear if this effect was due to changes in the film quality or due to an increase in the deposition rate as the RF bias increases the plasma density, as observed in Ref. 27. To determine the source of this effect, we characterized the deposition rate at each of the chamber conditions in question.
Figure 1details the effect of different chamber condi-tions, including changing N2flow rates and the addition of a 50 W RF bias, on the deposition rate and properties of NbN. Cross-sectional transmission electron microscopy (XTEM) images in Fig.1(a)show the structure and thickness of a film prepared with (top) and without (bottom) a 50 W RF bias applied during deposition. Less-than-100-nm-thick cross-sections were prepared using a gallium focused ion beam (FEI Helios Nanolab 600) and imaged using a field emission TEM (JEOL 2010F). Although both films appear to be poly-crystalline, the dark vertical stripes seen in the non-biased film are not evident in the film deposited with bias. The reduced surface roughness of the film prepared with bias is also evident. Figure 1(b) compares XRR measurements of the films imaged in (a). At incident angles greater than 2, interference fringes are more prominent in the film that was deposited with bias than without. Modeling and fitting the XRR curves using Rigaku’s GlobalFit software reveal that the root-mean-squared (rms) film roughness was reduced from 1.7 nm to 0.8 nm due to the addition of the RF bias, while the film density increased from 7.2 g/cm3to 7.5 g/cm3. Adding the bias during deposition reduced the roughness and increased density in all pairs of films measured. The rough-ness values obtained from XRR are about three times larger than those measured using an atomic force microscope (see SM for AFM images). Figure1(c)shows the deposition rate characterized by XRR under ten different deposition condi-tions. These films were deposited for 8 min, and measured thicknesses ranged from 46 nm for a film deposited with 2 cm3/min of N2 flow without bias to 23 nm for a film
deposited with 10 cm3/min of N2flow and a 50 W bias. On average, the addition of the bias reduced the film deposition rate by 12.8%. Top down TEM images of NbN prepared on SiO2TEM windows (seesupplementary material) show that
the NbN films prepared for this work were polycrystalline, regardless of the preparation method.
After characterizing the deposition rate under different deposition conditions as shown in Fig.1(c), we then depos-ited 5-nm-thick films by adjusting the deposition time. While the voltage drop across the plasma varied within the first one or two seconds after opening the shutter to begin the deposi-tion, this transient period was much shorter than the shortest deposition time of 52 s. Therefore, we believe that we are in a regime where the thickness is well approximated by the product of the deposition rate and time. After the conclusion of all of the depositions for this work, we repeated the depo-sition of the very first film considered and measured its thick-ness with XRR. We found that the deposition rate had fallen by 3.6%. This reduction of the deposition rate over time can be explained by a reduction in the voltage that develops between the plasma and the target due to an effective increase in the magnetron trap strength as the sputter target is thinned.29,30
In Fig. 2, we show that the addition of a 50 W RF bias decreased the resistivity and increased the superconducting transition temperature for all of the 5-nm-thick films relative to comparable films prepared without bias. As shown in Fig.2(a),
the addition of the bias reduced the resistivity of the 5-nm-thick films by between 23% and 42%. For the 5-nm-5-nm-thick films, the addition of the 50 W RF bias during deposition increased the film Tcby between 25% and 73%. The maxi-mum Tc of the bias-sputtered 5-nm-thick films is close to what we believe is optimal for high detection efficiency, low jitter SNSPDs. The relative increase in Tcwas nearly linear as a function of the N2flow rate, suggesting that the nitrogen content of the film contributed to low Tc in the thin film limit, possibly by promoting structural disorder that is over-come by ion bombardment. The resistivity and Tc of the thick films used to calibrate the deposition rates were also measured. For these films, resistivity was lowered in all cases by between 55% and 36% (see supplementary mate-rial). In contrast, theTcof the thick films prepared with bias
was not always higher than without. The maximum Tc for
the thick films with bias was about 500 mK (5%) lower than the maximum Tc for thick films deposited without bias.
These results corroborate previous reports on bias sputtering of NbN,27,31and it is clear that only in the nanometer thick-ness limit relevant for SNSPDs does the addition of ion bom-bardment substantially improve the filmTc.
This material work was motivated by the desire to improve SNSPD performance, so, after characterizing the bias sputtered films, we designed and fabricated 75 nm wide SNSPDs using 5-nm-thick NbN deposited with a 50 W RF bias and 6 cm3/min N2 flow onto oxidized silicon. The FIG. 1. (a) Cross-sectional transmission electron microscopy (XTEM)
images of reactive DC magnetron sputtered niobium nitride (NbN) with (top) and without (bottom) a 50 W RF bias applied to the sample holder dur-ing the deposition. The texture of the two films is noticeably different, with the addition of the bias reducing or eliminating columnar grains28present in the film deposited without bias. Both XRD and top down TEM images (see supplementary material) indicate that the films are polycrystalline. (b) Experimental x-ray reflectivity (XRR) as a function of twice the incident angle for films shown in (a). Modeling of the measured interference pattern allows us to extract the NbN film density, thickness, and roughness. The film thickness estimated from TEM images matched the values derived from XRR to within 2 nm. (c) The deposition rate, as determined by XRR, for NbN films deposited for 8 min onto SiO2substrates at different flow rates of N2, with and without bias, or else held constant. The addition of the bias reduced the measured deposition rate at a given N2flow, presumably due to resputtering of the deposited film. The points indicated are deposition rates estimated from the XRR measurements shown in (b) of the films imaged in (a). Repeated XRR thickness measurements of a given film varied by less than 0.1 nm.
FIG. 2. Room temperature resistivity (top) and superconducting critical tem-perature (bottom) for 5-nm-thick NbN films deposited under different cham-ber conditions. The deposition time at each N2flow rate and bias condition was adjusted so that the deposited film would be 5 nm thick. The addition of the bias reduced the room temperature resistivity and increased theTcof a film deposited with identical conditions. Repeated measurements of filmTc gave the same value as within 30 mK, with a standard deviation of 13 mK. The resistivity was calculated from the sheet resistance measured by a four point probe multiplied by the thickness (5 nm). Black error bars represent an upper-bound estimate of the error, based on a 3.6% growth rate reduction during the course of the entire experiment, combined with a 62 second error in the deposition time and a 61.5% error in the measured sheet resistance. The superconducting transition region is illustrated in the bottom graph by the colored bars which indicate the temperatures where the resistance was between 10% and 90% of its value at 20 K.
nanowires were fabricated in a standard meander pattern with the active area covering approximately 11 lm2. Turnarounds were optimized to avoid current crowding and maximize the switching current of the device.32Gold electri-cal contact pads were fabricated on top of the NbN film by photolithography. After gold pad fabrication, 10 nm of SiOx was evaporated onto the surface of the NbN to protect it dur-ing subsequent fabrication steps and enhance adhesion of the electron-beam resist. Hydrogen silsesquioxane (HSQ; 6% concentration) was spin coated onto the chip at 3 krpm. The HSQ was exposed to a 125 keV electron beam with an areal dose density of 3840 lC/cm2. After exposure, the HSQ was developed by submerging the chip in 25% tetramethylammo-nium hydroxide for 25 s. The HSQ pattern was then trans-ferred to the NbN by reactive ion etching in CF4at 50 W for 4 min.
Many of the promising applications for SNSPDs require picosecond timing resolution and the ability to detect single infrared photons in the telecommunication band. Therefore, we characterized the detection efficiency and timing jitter of the fabricated SNSPDs at 1550 nm wavelength. Figure 3
summarizes our results including saturated single photon detection efficiency at 1550 nm and a timing jitter of 38 ps. SNSPD performance was characterized at 2.5 K, in vacuum, on the coldhead of a liquid helium flow cryostat, with mea-surement conditions identical to those previously reported.33 The single-photon counting regime was confirmed by
measuring the count rate from the SNSPD, which was linear as a function of the applied laser power. Figure 3(a)shows the counting rate of one of these devices as a function of the device bias current. The timing distribution with respect to the laser pulse was measured, yielding a jitter of 38 ps (full width at half maximum). By fitting to the exponential decay of a photoresponse voltage pulse, we estimated the kinetic inductance of the material to be approximately 150 pH per square. The large kinetic inductance of the bias sputtered material was used to advantage in a recently demonstrated nanowire imager device22and was also used to fabricate the recently introduced nTron,21 whose DC performance is shown in Fig. 3(d). While we did not attempt to maximize the coupling efficiency to our SNSPDs nor the maximum count rate, we believe that the saturated detection efficiency, combined with the 38 ps timing jitter, makes this a promising material for combining high detection efficiency and low jit-ter in a single device that can operate at liquid helium temperatures.
In conclusion, we demonstrated that the addition of ion bombardment due to RF biasing of the substrate holder dur-ing the sputter deposition of few-nanometer-thick NbN films onto unheated SiO2 substrates increased Tc and reduced resistivity, and we used this material to fabricate SNSPDs with saturated detection efficiency and a jitter of 38 ps. A nanowire imager22and nTrons were also demonstrated using the same material. While the addition of the bias decreased the film resistivity in all cases, the filmTcwas not necessar-ily increased when deposited to tens of nanometers in thick-ness, indicating a decoupling of resistivity and critical temperature that could provide an experimental avenue for exploring a recently observed relationship between the thin film superconductor thickness, sheet resistance, and Tc.
34 This method of preparing NbN thin films at ambient temper-atures should be compatible with a wider array of fabrication techniques and materials than NbN that is deposited at ele-vated temperatures or onto lattice matched substrates and thus should enable fabrication processes that were previously not feasible. In particular, this method should allow easy integration of SNSPDs with existing photonic integrated circuits.35
Seesupplementary materialfor methods,Tc, and
resis-tivity data for thick NbN films, AFM, XPS, and XRD data, and top down TEM images of different preparations of NbN.
The authors would like to thank Jim Daley and Mark Mondol of the MIT Nanostructures lab for the technical support related to electron beam fabrication. They also like to thank Dr. Charles Settens from the Center for Materials Science and Engineering X-ray Facility for his assistance and advice on all matters related to x-ray measurements. This work made use of the Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award number DMR-1419807. Andrew Dane was supported by a NASA Space Technology Research fellowship, Grant No. NNX14AL48H. This research is based on work supported by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via contract W911NF-14-C0089. The FIG. 3. Measurements of devices made with bias sputtered NbN. (a)
1550 nm photon count rate of a 75 nm wide SNSPD fabricated from 5 nm thick bias-sputtered NbN as a function of bias current. The count rate appears to saturate, suggesting that the internal quantum efficiency of the SNSPD may be close to unity. The instrument response function is shown in (b), and the fit curve is in red, with a full width at half maximum of 38 ps. (c) Helium ion microscopy image of a representative SNSPD. (d) We also used bias sputtered NbN to fabricate the recently introduced nTron,21whose DC transfer characteristic was measured at the 4 temperatures shown. The NbN was approximately 10 nm thick, while the gate and channel widths were 30 and 400 nm, respectively.
views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon.
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